1. Field of the Invention
The present invention relates to circuits for automatically leveling a modulated signal. More particularly the invention relates to such circuits as comprise a variable gain amplifier, connected in a feedback network in which the gain of the amplifier is adjusted so that an output signal amplitude is automatically held to a desired reference value. The invention is applicable to the control of the level of a composite signal containing video, audio and synchronization information.
2. Prior Art
The present invention has practical application to a fixed point-to-point microwave communication system in which it is desired to couple a composite signal, which is locally available, to a microwave linked communication system for transmission to a remote location. In such systems it has been found desirable to provide a "modem" for standardizing the level of any locally available composite video signals before they are transmitted over the linkage. A conventional composite video signal, such as one required to meet a broadcast specification, consists of synchronization in the form of pulses for timing the fields, the frames and the lines of the video display, a color burst (for color signals), the video signal itself consisting of luminance or luminance and chrominance information (for color signals). An audio signal is also provided on a subcarrier at a frequency above the chrominance signal.
The composite video signal has a prescribed format aiding uniformity in displaying the eventual picture. A principal feature of this standardization is a prescribed relation between the amplitude of the video signal and the magnitude of the horizontal synchronization pulses. In addition, the phases within the video signal must be accurately maintained, particularly with color signals if the color rendition is to be correct. In this application, if the level of the composite signal is adjusted, the adjustment must occur without affecting the "differential gain" or the "differential phase" of the signal beyond a small tolerance. The differential gain requirement is a requirement that the gain of the amplifier (or attenuator) processing the signal be constant within a small tolerance for both low and high level signals.
Accordingly, in conventional microwave communications equipment for television signals, the modem, which is used to adjust the level of the composite video to a desired standard value, should provide adequate gain to maintain the composite video at said prescribed level over a reasonable range of levels available from the local video sources, and be able to do so without adversely affecting the quality of the composite video signal. In the application to which the present invention is addressed, the amplifier must have an adequate range of gain to bring the signals over this range of levels to the desired value, while at the same time meeting the differential gain and differential phase specifications required to maintain video signal quality.
In television equipment, the conventional automatic gain control circuit involves a variable gain amplifier, a feedback loop in which the horizontal synchronizing pulse is separated at the amplifier output, and compared to a standard level, to provide a control voltage used to control the gain of the amplifier. The present invention follows this general approach.
Generally, meeting the specifications for maintaining studio quality in known circuits of this application poses a significant technical challenge, particularly in a simple and low cost manner. The critical element, which poses the problem, is the amplifier in which the gain is varied.